import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-3,3,50)

y1 = 2*x +1
y2 = x**2

# plt.figure()
# plt.plot(x,y1)

plt.figure(num=3,figsize=(8,5))
plt.plot(x,y2)
plt.plot(x,y1,color = "red",linewidth = 1.0,linestyle = "--")
plt.xlim((-1,2))
plt.ylim((-2,3))
plt.xlabel("I am x")
plt.ylabel("I am y")

# 用ticks更换小标
new_ticks = np.linspace(-1,2,5)
# print(new_ticks)
plt.xticks(new_ticks)
plt.yticks([-2,-1.2,0.5,1.6,3],
            [r"$really\ bad$",r"$bad$",r"$normal$",r"$good$",r"$very\ good$"]) #  这里可以使用 latex 语法
            # r表示raw字符串

# gca = "get current axis"
ax = plt.gca()
ax.spines["right"].set_color("none")
ax.spines["top"].set_color("none")
ax.xaxis.set_ticks_position("bottom")
ax.yaxis.set_ticks_position("left")
ax.spines["bottom"].set_position(("data",0))
ax.spines["left"].set_position(("data",0))
plt.show()